ICS (IMS centralized service, see also TS 23.292) in 3GPP is aiming to study the architectural requirements and alternatives for the delivery of consistent services to the user mainly via the services centralized in IMS domain regardless of the attached access type, e.g. CS (circuit switched, CS) domain access or IP-CAN. It also considers how to support the service continuity when the user changes the access type from CS domain to IP-CAN or vice versa even when being engaged in a mid-call service, i.e. service continuity at domain transfer.
To deploy ICS for an existing CS network, the MSCs (Mobile Switching Center) need to be upgraded to have ICS functionality, i.e. to act as a User Agent (UA) for a CS user towards IMS domain, therefore to convert CS signalling to SIP towards IMS and vice versa
The deployment of ICS may differ from one network to another, depends on the business plans of the network operator or depending on the current configuration of the network. Some operators may choose migrating all CS users to ICS over a long period of time, so that they may want to start with a small step in order to minimize the efforts in network integration and development. Some operators may see no need to upgrade all CS users to ICS in 5 to 10 years, so that they would like to restrict the impact on the existing CS network. In both cases, the operators would like to minimize the impact on CS network due to ICS.
The current solutions foresee that the delivery of consistent services shall be achieved by replacing the standard MSC capable of circuit switched connection services with enhanced MSC. The term enhanced MSC is used in the present context to refer to network nodes like an MSC which are capable of the above mentioned inter-working functionalities. In those situations wherein existing legacy MSCs (with legacy we will refer to MSC which are not enhanced, or which are not capable of performing interworking between a radio communication network and an IP multimedia subsystem) are present and are responsible for attaching user terminals, it is not possible to provide delivery of consistent services since such legacy non enhanced MSC are not adapted to perform registration with the IMS or handle the corresponding messages. The current solution therefore foresees a replacement of all existing MSC in an area in order to provide delivery of consistent services in the same area.
However, as seen above, one problem related to the existing solutions is that it is not possible to have an ICS deployment having a minimum impact on an already deployed existing network.